The present invention relates to a process and apparatus for removing water from water-containing solids and, more particularly, to a process and apparatus for dewatering carbonaceous solids such as coal having a relatively high moisture content, for example, low rank coal.
Known resources of coal and other solid carbonaceous fuel materials in the world are far greater than the known resources of petroleum and natural gas combined. Despite this enormous abundance of coal and related solid carbonaceous materials, reliance on these resources, particularly coal, as primary sources of energy, has been for the most part discouraged. The availability of cheaper, cleaner burning, more easily retrievable and transportable fuels such as petroleum and natural gas has in the past placed coal in a largely supporting role in the energy field.
As a result, efforts are being extended to make coal and related solid carbonaceous materials equivalent or better sources of energy than petroleum or natural gas. In the case of coal, for example, much of this effort is directed to overcoming the environmental problems associated with its production, transportation and combustion. For example, health and safety hazards associated with coal mining have been significantly reduced with the onset of new legislation governing coal mining.
Furthermore, numerous techniques have been explored and developed to make coal more suitable for burning and more readily transportable. Accordingly, more attention is being given to certain handling considerations such as pipeline transportation of coal, to both physical and chemical coal cleaning (beneficiation) processes and to aspects of these processes which can make coal and related carbonaceous material more attractive as fuel materials.
Attention has also been directed to reducing the water content of low rank coals in order to enhance their value as fuels. Since much of the water contained in low-rank coals is chemically bound thereto and/or is interstitially held within the coal particles, mere thermal drying of such coals at the comparatively low temperatures which are typical of many thermal drying operations fails to achieve a high enough level of water removal to make the coals competitive with low moisture content, high rank coals.
Several attempts have been made in the past to upgrade low-rank coal by a variety of dewatering techniques. For instance, U.S. Pat. No. 4,511,363 describes a method of upgrading low rank coals in which crushed low rank coal is dehydrated in a rotary drum in a drying section, the dehydrated coal is subjected to dry distillation in a distillation section and finally, the dry-distilled coal is cooled in a cooling section and permitted to absorb the tar which is produced during the distillation step.
U.S. Pat. No. 4,504,274 describes a process for enriching high moisture content low grade coals in a fluid bed by contact with heated char.
U.S. Pat. No. 4,400,176 describes a process for reducing the water content of coal containing bound water by maintaining the coal at a temperature of from 220.degree.-500.degree. F. in the presence of water at a pressure sufficient to maintain at least a portion of the water in the liquid phase for a time sufficient to release at least a portion of the bound water, this treatment being accompanied by contacting the coal with an acidic material selected from the group consisting of carboxylic organic acids containing up to about 6 carbon atoms, phenol, phenolic acids and inorganic acids.
Other processes for removing water from coals are disclosed, for example, in U.S. Pat. Nos. 4,461,624 (immersion of coal particles in residium at elevated temperature); 4,322,219 (contacting coal particles with polyethylene oxide of at least 200,000 molecular weight followed by evaporation of the water); 4,057,399 (treating coal particles with a hydrocarbon at elevated temperature and pressure sufficient to maintain a liquid system); 4,018,571 (heating a mixture of coal particles and water between 300.degree. F. and the critical temperature of water at a pressure sufficient to maintain the water in the liquid state followed by cooling and separating the coal particles from the water by agglomeration with a hydrocarbon liquid); 3,985,516 and 3,985,517 (coating dried coal with a heavy hydrocarbon material to aid in the removal of moisture and prevent its reabsorption by the coal); 3,961,914 (coating coal particles with silicon dioxide resulting from the reaction of silicon tetrachloride vapor and water followed by increasing the temperature to drive off water vapor and hydrochloric acid); 3,327,402 (treating coal fines with organic solvents such as linear alkanols, acetone, methyl ethyl ketone, diethyl ketone and acetonitrile to remove water); and, 1,960,917 (coating coal particles with an aqueous oil emulsion and cause clinging water to drain away).
Some of the disadvantages associated with known processes of dewatering coal include their potential for environmental damage and/or high cost.